Most current aircraft possess a flight management system, for example of the FMS type, according to the acronym standing for the term “Flight Management System”. These systems allow help with navigation, by displaying information useful to pilots, or else by communicating flight parameters to an automatic piloting system.
Notably, a system of FMS type allows a pilot or some other qualified person, to input, in pre-flight, a flight plan defined by a departure point of the flight plan, an arrival point of the flight plan, and a series of transit points or waypoints, customarily referred to by the abbreviation WPT. All these points can be chosen from among points predefined in a navigation database, and which correspond to airports, radionavigation beacons, etc. The points can also be defined by their geographical coordinates and their altitude.
The inputting of the transit points can be done through a dedicated interface, for example a keyboard or a touchscreen, or else by transferring data from an external device.
A flight plan then consists of a succession of segments, or “legs” according to the terminology customarily employed in this technical field.
Other data can be entered into the flight management system, notably data relating to the aircraft's load plan and to the quantity of fuel on board.
When the aircraft is in flight, the flight management system precisely evaluates the position of the aircraft and the uncertainty in this item of data, by centralizing the data originating from the various positioning devices, such as the satellite based geo-positioning receiver, the radionavigation devices: for example DME, NDB and VOR, the inertial platform, etc.
A screen allows the pilots to view the current position of the aircraft, as well as the route followed by it, and the closest transit points, the whole on a map background making it possible to simultaneously display other flight parameters and landmark points. The information viewed allows notably pilots to tailor flight parameters, such as heading, thrust, altitude, rates of climb or descent, etc. or else simply to control the proper progress of the flight if the aircraft is piloted in an automatic manner. The computer of the flight management system makes it possible to determine an optimal flight geometry, notably in the sense of a reduction in operating costs related to fuel consumption.
FIG. 1 presents a summary chart illustrating the structure of a flight management system of FMS type, known from the prior art.
A system of FMS type 100 employs a man-machine interface 120 comprising for example a keyboard and a display screen, or else simply a touch-type display screen, as well as at least the following functions, described in the aforementioned ARINC 702 standard:                Navigation (LOCNAV) 101, for performing optimal location of the aircraft as a function of the geo-location means 130 such as satellite or GPS, GALILEO based geo-positioning, VHF radionavigation beacons, inertial platforms. This module communicates with the aforementioned geo-location devices;        Flight plan (FPLN) 102, for inputting the geographical elements constituting the skeleton of the route to be followed, such as the points imposed by the departure and arrival procedures, the waypoints, the airways (or air corridors);        Navigation database (NAVDB) 103, for constructing geographical routes and procedures with the aid of data included in the bases relating to the points, beacons, interception or altitude legs, etc.;        Performance database, (PRFDB) 104, containing the craft's aerodynamic and engine parameters;        Lateral trajectory (TRAJ) 105, for constructing a continuous trajectory on the basis of the points of the flight plan, complying with the performance of the aircraft and the confinement constraints (RNP);        Predictions (PRED) 106, for constructing an optimized vertical profile on the lateral and vertical trajectory. The functions forming the subject of the present invention affect this part of the computer;        Guidance (GUID) 107, for guiding the aircraft in the lateral and vertical planes on its three-dimensional trajectory, while optimizing its speed. In an aircraft equipped with an automatic piloting device 110, the latter can exchange information with the guidance module 107;        Digital data link (DATALINK) 108 for communicating with control centres and other aircraft 109.        
The flight plan is entered by the pilot, or else by data link, with the aid of data contained in the navigation database. A flight plan typically consists of a succession of segments, customarily referred to by the name “legs”, which are formed of a termination and of a geometry, for example a geometry of turning type, or else of great circle or rhumb line straight line type. The various types of legs are defined in the ARINC 424 international standard.
The pilot thereafter inputs the parameters of the aircraft: mass, flight plan, span of cruising levels, as well as a or a plurality of optimization criteria, such as the Cl. These inputs allow the modules TRAJ 105 and PRED 106 to compute respectively the lateral trajectory and the vertical profile, that is to say the flight profile in terms of altitude and speed, which for example minimizes the optimization criterion.
During the flight, it may turn out to be necessary, for a civil or military need, to shorten the flight plan by directly rejoining a point of the flight plan not corresponding to the first point to be rejoined, and to continue the initial planning onwards of the latter point. The pilot may also be led to quit the trajectory of a flight plan during the flight, for example following requests by the air traffic control bodies, either with the aim of circumventing an obstacle generated by unfavourable meteorological conditions, or simply with the objective of saving time or fuel consumption, etc. In such situations, it is necessary that the aircraft rejoin the flight plan, onwards of the instant at which the constraint no longer applies.
It is then desirable that the most realistic possible rejoining trajectory be determined, and taken into account by the FMS for the predictive computations, notably in respect of flight time and fuel consumption.
If the pilot desires to shorten the flight plan or reintegrate the flight plan at a selected navigation point, two possibilities are offered him according to the prior art, such as illustrated in FIG. 2.
FIG. 2a illustrates the initial trajectory 20 of the aircraft 10 according to the flight plan FP: the aircraft has just crossed the navigation point WPa and is steering towards the following point WPb of the flight plan, which it must pass without overflight, and then it must steer towards the navigation point WPc, which it must also traverse without overflight and then steer towards the point WPd.
FIG. 2b illustrates the “DirTo” function according to the prior art: it consists in asking the FMS to compute a direct trajectory 21 to the selected point, here WPc. The effect is the modification of the arrival vector or arrival “course” at the point and therefore of the trajectory which follows the point.
The “course” is defined as the angle made by the trajectory of the aircraft at a given point with respect to a direction of reference, typically North (which may be referenced as magnetic North or as true North). The transition for rejoining the desired point WPc amounts to making a turn in the “logical” direction of the point, the consequence of which is to modify the rest of the trajectory.
FIG. 2c illustrates the function “DirTo Course In” (or “DirTo Radial In”): here the FMS computes a rejoining straight line 22 as a function of a given course towards a given point. If the straight line is computed with the course advised by the FMS (corresponding to the initial trajectory for passing the point WPc), then there will be no trajectory modification after the point WPc. On the other hand in the present case the FMS does not compute any continuous trajectory flyable from the aeroplane up to the rejoining straight line 22: it is the pilot's responsibility to fly the trajectory that he desires manually, the trajectory cannot be flown in an automatic manner.
The problematic issue can be generalized to the computation of continuous trajectories between a departure point according to a departure course and an arrival point according to an arrival course, corresponding to an alignment constraint upon arrival at the point considered. Thus no function allowing the computation of continuous lateral trajectories such as these currently exists in an FMS.
An aim of the present invention is to alleviate the aforementioned drawbacks, by proposing a method of computing continuous geometric trajectories of an aircraft between a departure point and a departure course and an arrival point while complying with an alignment constraint so as to arrive at this point.